Engines combust a mixture of air and fuel (air/fuel) to drive a piston in a cylinder. The downward force of the piston generates torque. The torque output can be increased by optimizing various factors, including volumetric efficiency of the engine. Volumetric efficiency can be defined as air flow delivered into the cylinder compared to a theoretical maximum air flow that could enter the cylinder. In theory, combusting the maximum amount of air/fuel within the cylinder delivers the largest amount of drive torque to the crankshaft.
Intake valves regulate the mass flow of air delivered to the cylinder. Exhaust valves regulate the amount of exhaust gas released into the exhaust system. The crankshaft drives camshafts that include cam lobes that mechanically actuate the valves. Valve overlap occurs when the intake valve and exhaust valve are simultaneously open. A large valve overlap can increase drive torque. By varying the level of air that enters an engine at high engine speeds, overlap forces exhaust gas out of the exhaust valve at a faster rate and allows increased amounts of fresh air to travel through the intake valve into the cylinder. At low engine speeds, large overlap may cause poor idle quality and may increase emissions.
The range of overlap can be adjusted by varying the opening and closing times of the intake valve and the exhaust valve. In fixed camshaft systems, the valve timing can be adjusted by designing cam lobe profiles that produce a desired overlap range.
In variable valve camshaft systems, the timing of valves is varied at pre-determined speeds using cam phasers that adjust the rotational position of the camshafts. Typically, a control module refers to a static reference table that includes predetermined cam phaser positions corresponding to various operating points. Traditionally, an operating point is defined as operating a vehicle at a predetermined constant speed, while simultaneously delivering a predetermined constant level air flow to the engine. As the engine operates at various operating points, the valve overlap is adjusted. Differences in manufacturing during production, geometric design and aging may hinder the ability to maximize the level of air flow into the engine.